The invention relates to a monolithic integrated semiconductor circuit with a plurality of low on-state resistance switching transistors in accordance with bipolar technology, particularly for use in switching networks of telephone exchange devices and transmission devices, which permit connection of a series of input lines to one or several output lines as desired.
From the magazine "Bulletin Technique" PTT 2 (1973), p. 79-83, a completely integrated matrix cross-point is known which is based on the conventional transistor crosspoint. It comprises a bipolar switching transistor in the series branch and a fixed resistor in the shunt branch, across which the switching transistor is connected and disconnected by means of a trigger circuit. In order to construct it in monolithic integrated form, it was so modified that a NPN-transistor is used as a switching transistor and a trigger circuit having a PNP-lateral transistor as a current source is used instead of the fixed resistor in the shunt branch and a NPN-transistor is used as a control. Both transistors of the trigger circuit are connected by their collectors to the base connection of the switching transistor. The NPN-transistor of the trigger circuit is in turn controlled bistably by means of a flip flop. If the NPN transistor is conductive then the switching transistor is blocked. If however the NPN transistor is blocked then the switching transistor is made conductive by the PNP-lateral-transistor. This circuit, in conventional bipolar monolithic technology has the disadvantage that the PNP-lateral-transistor is constantly current carrying, regardless of whether the switching transistor is blocked or conductive. As a result there is always a relatively high dc dissipation in the crosspoint. Consequently the packing density of the semiconductor crosspoint is limited in a matrix switching package established with the semiconductor crosspoints because the power dissipation in the spatially small matrix switching package is a substantial influence as:
1. The reliability of the semiconductor crosspoints is a function of the crystal temperature of the semiconductors and, PA1 2. The dc dissipation per package is limited to approximately 300 mW and thus determines the scope of integration per unit. PA1 (a) a first selective layer of a second type of conductivity opposite to a first type of conductivity is introduced into a substrate of said first type of conductivity, PA1 (b) a less heavily doped second layer of said second type of conductivity is epitaxially grown on to said first layer, PA1 (c) a third layer of said first type of conductivity is selectively introduced into said second layer, PA1 (d) a fourth layer having as large an area as possible of said second type of conductivity is selectively introduced into said third layer, PA1 (e) said second, third and fourth layers, which form said vertical NPN transistor, are enclosed by a region of said second conductivity type which together with said first layer forms a largely tank shaped region of said second type of conductivity which surrounds said transistor, PA1 (f) said tank shaped region is surrounded by a first frame-like region of said second type of conductivity, PA1 (g) said first frame like region is surrounded by a second frame like region of said first type of conductivity, said region extending as far as the substrate, being more highly doped than the substrate and serves to insulate said vertical NPN transistor, PA1 (h) said substrate, said tank shaped region, and said third and fourth layer are provided with connections; and wherein said connection of said tank shaped region and said connection of said fourth layer are connected to the lines to be switched through, said connection of said third layer is connected to a triggering circuit and, as desired, to a control circuit, and said connection of said substrate is connected to a dc voltage potential which is negative with respect to said tank shaped region.
A matrix switching package therefore has only 2 .times. 2 symmetrical semiconductor crosspoints, including the control logic circuit, on the same semiconductor chip.
Furthermore, the crosspoint known from the magazine has the disadvantage that the switching transistor requires a directed collector biasing current which limits the freedom of speech path connection in the switching network to connections from one side to the other and demands an expensive junctor circuit with lowering of current sinks and capacitatively or inductively coupled switching. A relatively large power dissipation in the junctor circuits is the result. The collector current is exceptionally high (I.sub.C .apprxeq.10 mA) and flows across a switching transistor into the junctor circuit.
As a result of the solution stated in German Offenlegungsschrift No. 23 63 699, (U.S. Pat. No. 3,931,475, issued Jan. 6, 1976, inventors: Bachle, Kliem and Rall) the stated disadvantages of this monolithic circuit known from the magazine are largely avoided. With this crosspoint a small shunt attenuation is achieved in an advantageous manner as a result of the control transistor operating in the active region when the switching transistor is in the on-state and when this cross-point is blocked, the control transistor is also blocked in an advantageous manner. In order to increase the shunt attenuation when the control transistor is blocked and to optimise the blocking action of the switching transistor, a voltage-dependent resistor is connected to the base of the switching transistor in the trigger circuit, said resistor being controlled by the base potential of the switching transistor so that, when the switching transistor is conductive, it operates in the high value resistance range and, when the switching transistor is blocked, it operates in the low value resistance range. In fact, the voltage-dependent resistor comprises a transistor which is provided with negative feed back to the emitter by means of a resistor; the base-emitter path of the transistor is constantly current-carrying. This crosspoint has in fact the advantage of a substantially smaller dc dissipation than the solutions previously stated, however a switching package with electronic crosspoints which, as is known, requires a trigger circuit, a flip flop, which gives the cross-point its bistable character, and a control logic circuit besides the switching transistor, still has a very large area and is therefore expensive.
The power dissipation of a 5 .times. 2 switching package which may be manufactured by normal bipolar technology and used for symmetrical switching, i.e. a switching package having 20 switching transistors, is at approximately 300 mW and its area on a silicon chip is approximately (4.5 .times. 4.5) mm.sup.2, if a maximum of four switching transistors are in the on-state. Thus the NPN switching transistor takes up an area of approximately (200 .times. 120) .mu.m.sup.2 when there is an on-state resistance of approximately 13 .OMEGA.(direct current of the base I.sub.B = 2 mA).
A switching transistor manufactured in previously known bipolar technology with an on-state resistance of approximately 5 .OMEGA. requires an area of (400 .times. 400) .mu.m.sup.2 even with a base current of 2 mA.